Showy milkweed establishment by seed, rhizome, and transplants in California’s Central Valley

Milkweed species provide breeding sites and food essential for the survival of monarch butterflies (<i>Danaus plexippus</i> L. [Lepidoptera: Nymphalidae]) that overwinter on the California Coast and migrate inland to other western states. Showy milkweed (<i>Asclepias speciosa</i> Torr. [Asclepiadaceae]) is widely distributed throughout Northern California; however, populations have declined for a variety of reasons including discontinuation of indigenous stewardship methods, a transition to rangelands dominated by weedy annual grasses, and an increase in intensive agriculture. Although seed and greenhouse materials are commercially available, establishment rates are discouraging. We evaluated field establishment of plants grown from seed, rhizomes, and 3 planting times for transplants (fall, early spring, and late spring) in California’s San Joaquin Valley. Evaluations were conducted during 2018 and 2019 for germination and survival, plant height, disease damage, and insect damage. Establishment and survival were rated as moderate to excellent for all planting methods. Showy milkweed rhizomes had the highest rated establishment and survival rates in both the 2018 and 2019 growing seasons and were tallest in height. All fall and spring transplants showed good establishment during both years of the trial. Despite having the lowest establishment scores based on percentage of propagules established, seeding in the fall had the highest estimated plant densities after 2 y; however, seeded plots required significant hand weeding. Insect and disease damage was significant among all planting methods.